1. Field of the Invention
The present invention relates to control systems for charging a steam boiler, and in particular to control systems for charging a wood-waste burning steam boiler in response to steam demand and pollution output.
2. Description of the Prior Art
Large quantities of wood-waste particulate materials, such as saw dust, wood chips, hogged bark, tan bark, and the like are produced in many industries. Disposal of such particulate material in a safe, efficient and pollution-free manner is a problem faced by all such industries. Waste material of this type is generally stored in large containers equipped to discharge the waste material utilizing various unloading mechanisms. Some known bottom unloaders are comprised of a rotating screw-type conveyor having means for driving the conveyor screw around the bottom of the container in a radially sweeping motion. Driving the conveyor screw in this manner forces the conveyor screw through the particulate material regardless of whether the conveyor is moving material at its maximum capacity, thus resulting in less than maximally efficient operation. Such prior art conveyor devices also tend to arch upwardly into the particulate material in the container as a result of the natural tendency of the rotating screw to "climb," ultimately resulting in bending or breaking of the conveyor screw.
A particularly suitable apparatus for unloading particulate material from the bottom of such containers is disclosed and claimed in U.S. Pat. No. 3,414,142 granted to M. W. Kolze (which is incorporated herein by reference). The system disclosed and claimed in U.S. Pat. No. 3,414,142 generally comprises a non arching-radially sweeping conveyor screw for bottom unloading of particulate material from a storage container into a preparatory means which includes a sonic sensor for maintaining an optimum amount of particulate material in the preparatory means. A control system integrated with the charging apparatus controls mixing of the compacted particulate material with a controllable volume of air provided by a blower and also controls damper valves to feed an optimum amount of particulate material-air mixture into a combustion chamber, such as a boiler, for burning therein. The integrated control system receives feedback information relating to critical parameters within the boiler, such as boiler temperature, water supply, and smoke and pollution level and varies the operation of the charging system in accordance with the received feedback information so that an optimum amount of particulate material is fed into the combustion chamber for substantially complete combustion of the material and minimization of pollutants.
A problem in the art is that operation of a conveyor screw below a discharge bin with an overloading of saw dust above such a screw will, because of the tendency of saw dust to compact due to its own weight, result in a self-supporting mass of material which can be tunnelled through by the conveyor screw thus leaving a supporting arch of saw dust above the screw. After the conveyor screw tunnels out the saw dust beneath the supporting arch, no further saw dust will be transported even though a measurement of saw dust levels would indicate that sufficient saw dust is present in the bin. In fact, detection of the problem is possible only after, for example, a saw dust burning machine fed by the empty conveyor screw cycles down for lack of saw dust input. The manual intervention of an operator is then required to dislodge the compacted saw dust.
If too much saw dust or other wood-waste material is supplied into a combustion chamber, incomplete combustion will result and cause extensive and unacceptable smoke, ash and other pollution. If insufficient saw dust is fed into a combustion chamber, however, an inefficient combustion and/or disposal of material occurs.
The same problems are present to a greater or lesser degree with the use of other types of wood-waste materials as fuel.
An apparatus for overcoming some of the above problems is claimed and disclosed in U.S. Pat. No. 3,865,053 granted to B. A. Kolze and M. W. Kolze (which is incorporated herein by reference). That apparatus utilizes the unloader of U.S. Pat. No. 3,414,142 to adjustably control the input of wood-waste material to a conveyor screw which feeds the material to a choke screw which transports a selected amount of material to a selected volume of air from a rotating blade which pulls the material from the choke screw and feeds the mixture into a combustion chamber of a boiler for burning. The system includes control circuitry for integrating the functions of the entire system so that an optimum amount of particulate matter is fed to the combustion chamber.